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1949 SCHLIEREN PHOTOGRAPHY 539
In addition, it is questionable if sufficient density could be obtained from such a small source to render a usable photograph.
Fig. 13 indicates that for ballistic and wind-tunnel studies, where shock waves are the important disturbances to be photographed, source sizes of the order of 1 mm in width are satisfactory. Experience indicates that spark photographs taken with Charters or Liebessart-type sparks at energies of the order of 5 watt-seconds have excellent quality, for source sizes of 1-mm diameter.
When working with sources of finite size the contrast and density in the image can be controlled by varying the knife-edge position as shown in Figs. 14 and 15.
1.6
1.4
i/O 1.2
| 1.0
'<O
-4 2
CONTRAST
— DENSITY
, A/a10
R
g 7 6 53
,, C/">
4 — — 1
a -<
2
i
io' 10* ia2 10" i 10 10* io
\jj or iy — wim
Fig. 13 — Effect of source size on the contrast and density of the images of two disturbances of Fig. 5. R is the radius of the Airy disk of the aperture A -A, m is the position of the central maxima of the Airy disk for region a-a when A/a = 10, n is the position of the central maxima of the Airy disk for A/a = 104.
As the knife-edge is moved into the diffraction pattern from — °° , the contrast varies more rapidly than the density. For the 1.25-mm source of Fig. 14 there is a small region in the vicinity of the optic axis for which the knife-edge position is not critical. As the cutoff is further increased, the density varies slowly. This is because of the decrease of light in the background at a much higher rate than in the center of the image. While higher cutoff will give very great contrast between the image of the region a-a of Fig. 5 and the background, small disturbances in the background will not be readily visible. As a general rule it is desirable to operate with the knife-edge in such a position that there is also a marked degree of contrast between the